TITLE:   GCN CIRCULAR
NUMBER:  36945
SUBJECT: GRB 240727B: Swift/BAT-GUANO arcminute localization of a long burst
DATE:    24/07/27 12:33:16 GMT
FROM:    Samuele Ronchini at PSU <sjs8171@psu.edu>

Samuele Ronchini (PSU), James DeLaunay (PSU), Aaron Tohuvavohu (U Toronto), Gayathri Raman (PSU), Jamie A. Kennea (PSU), Tyler Parsotan (NASA GSFC) report:

Swift/BAT did not localize GRB 240727B onboard (T0: 2024-07-27T06:00:08.96 UTC, AstroSat GCN 36943, Fermi trig. 743752813).

The Fermi notice, distributed in near real-time, triggered the Swift Mission Operations Center operated Gamma-ray Urgent Archiver for Novel Opportunities (GUANO; Tohuvavohu et al. 2020, ApJ, 900, 1). 

Upon trigger by this notice, GUANO sent a command to the Swift Burst Alert Telescope (BAT) to save 200 seconds of BAT event-mode data from [-50,+150] seconds around the time of the burst. All the requested event mode data was delivered to the ground.

The BAT likelihood search, NITRATES (DeLaunay + Tohuvavohu 2022, ApJ, 941, 169), detects the burst in a 16.384 s analysis time bin starting at T0 s with a sqrt(TS) of 45.6.
An arcminute localization is found with DeltaLLHOut of 29.3 and a DeltaLLHPeak of 49.37.

See Section 9.1 and Figures 10 and 17 in the NITRATES paper for brief descriptions and interpretations of sqrt(TS), DeltaLLHPeak, and DeltaLLHOut.

The BAT position is
RA, Dec = 97.786, -37.18 deg which is
   RA(J2000)  = 06h 31m 08.64s
   Dec(J2000) = -37d 10′ 48.0″
with an estimated uncertainty of 4 arcmin radius.

XRT and UVOT follow-up has been requested.
Results of follow-up observations will be reported in future circulars.

GUANO is a fully autonomous, extremely low latency, spacecraft
commanding pipeline designed for targeted recovery of BAT event mode
data around the times of compelling astrophysical events to enable
more sensitive GRB searches.

A live reporting of Swift/BAT event data recovered by GUANO can be
found at: https://www.swift.psu.edu/guano/